MM subisoenzymes of creatine kinase as an index of disease activity in polymyositis

Creatine kinase (CK; EC 2.7.3.2), although the most commonly measured enzyme for assessing disease activity in polymyositis, is not always a reliable indicator of the extent and severity of myositis. Recently, the CK-MM isoenzyme has been found to undergo post-synthetic modifications upon release into the serum, such that electrophoretically identifiable sub-bands or subisoenzymes - MM₁, MM₂, and MM₃ - are produced. To determine the diagnostic and discriminative value of these subisoenzymes in polymyositis, we analyzed CK and its MM subisoenzyme forms in serum samples from 22 patients with myositis and from 23 controls. In the presence of inflammatory myositis and increased total CK activity, MM patterns correlated with the clinical trend, often more accurately than did measurements of total CK. MM₁ proportions > 30% of total CK-MM or ratios of MM₃ to MM₁ < 1 were associated with an improving or stable condition, whereas MM₁ activity < 30% or MM₃/MM₁ > 1 reflected a deteriorating course of disease. Patients whose disease was assessed to be clinically deteriorating were clearly distinguished from patients with improving disease by their subisoenzyme patterns (p < 0.01). Thus these patterns add significantly to the information obtainable by routine blood analysis.